The present invention relates to a micro-electromechanical optical attenuator and a method of operating the same.
A micro-electromechanical (MEMs) optical attenuator is an electrostatically driven tilting micromirror device within a lens system that directs an optical signal onto the micromirror surface. Attenuation of the optical signal is obtained by applying an electrical signal to the micromirror device causing its reflecting surface to tilt, and thus deflecting a portion of the optical signal away from the lens system. Incomplete coupling of the signal results from the deflection. Thus, the amount of deflection controls the amount of attenuation. It is desired to control the level of attenuation precisely.
However, control of the attenuation is difficult for two reasons. The attenuation obtained from a deflection angle does not change uniformly over a range of interest. In addition, over a range of interest the required voltage to obtain a desired deflection changes as a non-linear function. Prior art attenuators of this type suffer from poor control because, viewed graphically, the attenuation as a function of deflection, and the angle of deflection as a function of voltage combine to produce a very unstable voltage to attenuation response function. Within the deflection range of interest a large change in voltage may cause a relatively small change in attenuation, and within the same range a small change in voltage may cause a large change in attenuation.
Reflective attenuators are also used in an array to construct a dynamic gain equalizer (DGE) which works on the same principle and suffers from the same unstable response. A dynamic gain equalizer is used to equalize the gain for all channels in an optical amplifier. Typically a DWDM/WDM system will use an amplifier to regenerate the optical signals in all channels. However, the gain is usually not equal for each channel. By attenuating the channels with too much gain, the gain over all channels can be equalized. Individual channels are demultiplexed and simultaneously directed to a DGE comprising an array of optical attenuators for selective gain equalization.
There is a need to provide a micro-electromechanical (MEMs) attenuator or a dynamic gain equalizer with a more stable controlled response.
The present invention has found that by operating the electrostatic drive of an attenuator such that a low loss state is attained with the mirror in a fully deflected position, the drive voltage can be decreased to increase attenuation in a very stable nearly linear attenuation vs. drive voltage response. The non-linearity of the attenuation vs. deflection function is countered by a deflection vs. voltage response that becomes increasingly stable with decreasing voltage. A resulting attenuation vs. voltage curve gives a superior, nearly linear response.
Accordingly, the present invention provides a method for controlling a reflective attenuator in an optical system, having a lens system for directing a beam of light and a mirror for reflecting the directed beam of light, the mirror having a drive for selectively deflecting the mirror to achieve a desired attenuation comprising the steps of:
establishing a maximum deflection angle of the mirror corresponding to a maximum desired attenuation value in the optical system;
supporting the mirror drive in an unpowered state to position the mirror at the maximum deflection angle;
applying a voltage to the mirror drive to drive the mirror to a position to achieve optimum coupling; and
decreasing the applied voltage to the drive system to permit the mirror to return to a deflection position between optimum coupling and the maximum deflection angle to obtain a selected attenuation.
In a further embodiment the present invention provides a reflective optical attenuator comprising:
a lens system for directing a beam of light;
a mirror for reflecting the directed beam of light at least partially back to the lens system, the mirror having a drive system for selectively deflecting the mirror;
a support for supporting the drive system in an unpowered state to position the mirror at a selected maximum deflection angle;
wherein an increasing voltage applied to the drive system decreases the attenuation until optimum coupling is reached, and releasing a voltage applied increases attenuation until the selected maximum deflection angle is reached.
Advantageously, the present invention provides an attenuator that has a nearly linear response enabling stable deflection to achieve a desired attenuation. In accordance with the present invention, the attenuator requires very low voltage. In the optimum coupling to low loss state the attenuation vs. voltage response is quite insensitive, thus a small tilt offset in the system does not affect performance.